Postexposure baking

Acid generation in photoresist films add photogeneration vs. dose, 3233/ acid present after irradiation, 32,34r add present before irradiation, 32 quantum yield, 3234 Acid hardening resin resists cross-linking adivation energy determination, 87,89 cross-linking chemistry, 87 determination of acid generated, 87-88 effect of postexposure bake temperature and time, 87... 

Entry Postexposure Baked Temperature (°C) Optimum Exposure (mj / cm2) Maximum Resolution (nm) Mask Fidelity (nm) Line Edge Roughness (nm)... 

As discussed previously, an optional postexposure, predevelopment bake can reduce problems with the standing-wave effect in DNQ-novolac positive resists. However, such a postexposure bake step is indispensable in the image reversal of positive resists (37-41) and certain resists based on chemical amplification of a photogenerated catalyst (64-67, 77, 78). For both types of resists, the chemistry that differentiates between exposed and unexposed areas does not occur solely during irradiation. Instead, differentiation occurs predominantly during a subsequent bake. Therefore, to obtain acceptable CD control in these systems, the bake conditions must be carefully optimized and monitored. 

Scheme 1 Acid-catalyzed deprotection reaction. In the postexposure bake, the acid generated by the photoacid generator molecules (PAG) in a photochemical reaction catalyzes a deprotection reaction that cleaves the pendant group of the insoluble polymer, resulting in a polymer that is soluble in the developer...

Brominated poly(l-trimethylsilylpropyne) is an example of a substituted polyacetylene that is suitable for bilevel-resist processes (34). Requiring both exposure and postexposure bake (PEB) steps, samples of the polypropyne having a mole fraction of bromine from 0.1 to 0.2 per monomer unit exhibit sensitivities in the order of 25 mj/cm. Submicrometer resolution has been demonstrated, and etching-rate ratios relative to hard-baked photoresist planarizing layers are —1 25. 

Figure 8. Sensitivity curves for brominated derivatives of poly(TMSP) after postexposure baking for 1 h at 140 °C. Sample designations are explained in the caption to Figure 2.

Postexposure bake of the wafer. A postexposure bake (PEB) improves contrast of the photoresist before its development. The PEB process causes three effects 1) diffusion of the PAC 2) solvent evaporation and 3) thermally induced chemical reactions. In general, the dissolution rate of a resist decreases as a function of a PEB temperature. PEB becomes more important for the photoresists with a chemical amplification (CA) feature. The photoresists need the PEB to complete chemical reactions initiated by exposure. 

In 1979, Frechet and Willson put forward a very productive idea of a chemical amplification that was used in the development of a new generation of photoresists.They decided to use a photoresist comprising of a photochemical acid generator (PAG) and a polymer that was able to switch from hydrophobic to hydrophilic in the course of acid catalyzed hydrolysis. The PAG reacts with light to produce an acid catalyst. During a subsequent postexposure bake, the catalyst diffuses and reacts with the polymer component, causing many reaction events in the polymer and recovers the acid catalyst. The acid molecules catalyze the deprotection reaction and provide a prerequisite for chemical amplification. The number of the reaction events initiated by single quantum absorption has been estimated to be of order of 100. ... 

Figure 1. Effect of postexposure baking on vapor development rate for single pass and multiple pass exposures.

Figure 6. Fraction of film remaining (expressed as relative absorbance) vs. dose. Key o and aliphatic peak and and a., sulfone peak. Key( aliphatic and sulfone peak) , y-irradiated sample and C , after postexposure baking (110 °C).

Figure 11. Vapor development curves for PMPS in 20 PMPS/80 novolac composite film (lower curve postexposure baked at 130 °C for 2 h). Key , 1298 cm-1 ...

Commercially available water soluble copolymers of maleic anhydride with ethylene and methyl vinyl ether (presumably partially hydrolyzed to vicinal carboxylic acids) and triphenylsulfonium triflate were dissolved in water as a casting solvent. Spin-cast films were baked at 130 °C for 10 s, exposed to 254 nm radiation, and postexposure-baked at 130 °C for 40 s. Development with pure water provided negative tone images, presumably due to acid-catalyzed dehydration between vicinal carboxylic acids to form less polar anhydride, as was demonstrated for polarity reversal (4.3.1). However, a copolymer of maleic acid with methyl vinyl ether failed to provide any negative images,... 

In general, the chemical transformations associated with the chemical amplification mechanism in resists is effected through heating the exposed resist film, in a process called postexposure bake (PEB). Although, in principle, the active catalytic species (ions or radicals) could be generated from either photochemical (or radiochemical) acid or base generators, the acid generators are now used almost exclusively in advanced resist systems. ... 

Figure 11.13 SEM images of line-and-space patterns printed with 60-nm-thick Shipley XP-98248 resist on bare silicon and exposed at 157 nm. Process conditions postapplied bake 130°C/60 seconds, postexposure bake 130°C/90 seconds, developer 0.26N tetramethylammonium hydroxide (without surfactant) for 20 seconds. Unexposed resist loss 6nm. Exposure energy 1.35 mJ/cm. Note the significant surface inhibition layer, showing poisoning effects. ...

Lithographic modeling simulates several key steps in the lithographic process comprising image formation, resist exposure, postexposure bake diffusion,... 

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